In many memory devices, including random access memory (RAM) devices, data is typically accessed by supplying an address to an array of memory cells and then reading data from the memory cells that reside at the supplied address. However, in content addressable memory (CAM) devices, data within a CAM array is typically not accessed by supplying an address, but rather by applying data (e.g., search words) to the array and then performing a search operation to identify one or more entries within the CAM array that contain data equivalent to the applied data and thereby represent a “match” condition. In this manner, data is typically accessed according to its content rather than its address. Upon completion of the search operation, the identified location(s) containing the equivalent data is typically encoded to provide an address (e.g., block address+row address within a block) at which the matching entry is located. If multiple matching entries are identified in response to the search operation, then local priority encoding operations may be performed to identify a location of a best or highest priority matching entry. Such priority encoding operations frequently utilize the relative physical locations of multiple matching entries within the CAM array to identify a highest priority matching entry. An exemplary CAM device that utilizes a priority encoder to identify a highest priority matching entry is disclosed in commonly assigned U.S. Pat. No. 6,370,613 to Diede et al., entitled “Content Addressable Memory with Longest Match Detect,” the disclosure of which is hereby incorporated herein by reference. Additional CAM devices are described in U.S. Pat. Nos. 5,706,224, 5,852,569 and 5,964,857 to Srinivasan et al. and in U.S. Pat. Nos. 6,101,116, 6,256,216, 6,128,207 and 6,262,907 to Lien et al., assigned to the present assignee, the disclosures of which are hereby incorporated herein by reference.
CAM cells are frequently configured as binary CAM cells that store only data bits (as “1” or “0” logic values) or as ternary CAM cells that store data bits and mask bits. As will be understood by those skilled in the art, when a mask bit within a ternary CAM cell is inactive (e.g., set to a logic 1 value), the ternary CAM cell may operate as a conventional binary CAM cell storing an “unmasked” data bit. When the mask bit is active (e.g., set to a logic 0 value), the ternary CAM cell is treated as storing a “don't care” (X) value, which means that all compare operations performed on the actively masked ternary CAM cell will result in a cell match condition. Thus, if a logic 0 data bit is applied to a ternary CAM cell storing an active mask bit and a logic 1 data bit, the compare operation will indicate a cell match condition. A cell match condition will also be indicated if a logic 1 data bit is applied to a ternary CAM cell storing an active mask bit and a logic 0 data bit. Accordingly, if a data word of length N, where N is an integer, is applied to a ternary CAM array having a plurality of entries therein of logical width N, then a compare operation will yield one or more match conditions whenever all the unmasked data bits of an entry in the ternary CAM array are identical to the corresponding data bits of the applied search word. This means that if the applied search word equals {1011}, the following entries will result in a match condition in a CAM comprising ternary CAM cells: {1011}, {X011}, {1X11}, {10X1}, {101X}, {XX11}, {1XX1}, . . . , {1 XXX}, {XXXX}.
CAM devices may also include multiple match detection circuits that generate a flag to indicate the presence of multiple matching entries when a search operation is performed within a CAM device. For example, FIG. 3 of U.S. Pat. No. 6,175,513 to Khanna illustrates a multiple match detection circuit 10 that uses logic 15 to generate a plurality of intermediate multiple match flags (MM1-MM3). A final multiple match flag (MMF) is generated by logically combining the intermediate multiple match flags using an OR gate. The speed and scalability of the multiple match detection circuits disclosed in the '513 patent may be limited by the fact that each match signal input sees a relatively large fanout. For example, as illustrated by FIGS. 5 and 6A-6E of the '513 patent, a fanout of n (e.g., 5) will be required in a multiple match detection circuit that receives 2n (e.g., 32) match signal inputs. U.S. Pat. Nos. 6,924,994 and 6,859,378 to Lin et al. also disclose CAM devices with multiple match detection circuits therein that are scalable. Additional multiple match detection circuits are disclosed in U.S. Pat. Nos. 5,748,070, 5,852,569, 6,195,277, 6,307,798 and 6,392,910.